The present invention relates to a coating apparatus for coating a flexible support with coating composition in order to form a magnetic recording medium, a photographic sensitive material, an electronic material, a coating-type battery, an optical film for anti-reflection or the like, abrasive tape, information recording paper, etc.
Coating apparatus for simultaneously forming a single layer or a plurality layers of thin coatings on a flexible support of plastic or the like at a high speed are disclosed in JP-B-5-8065, JP-B-6-77712, and so on.
In producing a magnetic recording medium or the like, it is requested to apply coating composition such as magnetic liquid or the like onto a flexible support to a predetermined thickness with precision. To this end, JP-B-7-106332 proposes a coating apparatus which can adjust the coating thickness of coating composition. In this coating apparatus, the width of a coating composition supply channel (slit) is adjusted to change the loss of pressure inside the channel so that the discharge amount of coating composition is adjusted. According to this coating apparatus, it is possible to form a coating with a substantially uniform thickness over the whole area of a wide flexible support which is running.
On the other hand, Japanese Patent No. 2,691,602 proposes a method in which the swell of each edge surface of an extrusion-type coating apparatus is reduced in order to restrain coating failures such as coating streaks or the like.
In the coating apparatus disclosed in JP-B-7-106332, however, it was impossible to adjust the variation in coating thickness in the width direction of a product having a width of a few millimeters, such as a magnetic tape. In addition, when the adjusted amount of the slit width was large in this coating apparatus, the relationship of height between the front edge surface and the doctor edge surface changed so that the shape of a liquid reservoir was transformed. As a result, there might arise coating failures such as coating unevenness, coating streaks or the like, other than thickness variation.
Further, if the swell of each edge surface of an extrusion-type coating apparatus was merely reduced as proposed in Japanese Patent No. 2,691,602, there might still arise coating failures.
The present invention was developed on the basis of the above-mentioned background. It is an object of the present invention to provide a coating apparatus which can apply coating composition onto a flexible support without producing any variation in thickness or any coating failure.
The above object of the present invention can be achieved by the following configurations.
(1) An extrusion-type coating apparatus having a configuration in which a coating head having a front edge surface and a doctor edge surface is pressed onto a flexible support laid so as to run between pass rollers so that the coating head applies coating liquid discharged from a slit onto the flexible support; wherein blade thickness of the doctor edge surface in a direction of running of the support has a rate of variation not exceeding 5% in a direction of width of the support.
(2) A coating apparatus according to the above item (1), wherein the doctor edge surface has a curved surface, and a deviation of a curvature center of the curved surface in the direction of the blade thickness does not exceed 5% of blade thickness of the curved surface.
(3) A coating apparatus according to the above item (1) or (2), wherein a deviation of width of the slit does not exceed 5 xcexcm.
(4) A coating apparatus according to any one of the above items (1) to (3), wherein a deviation of a difference of height between adjacent edge surfaces of the coating head does not exceed 5 xcexcm.